In diagnosing urinary tract infections (UTIs), the gold standard is to perform a
ID: 3302069 • Letter: I
Question
In diagnosing urinary tract infections (UTIs), the gold standard is to perform a culture test. A new more rapid test, the dipstick test, is being considered for use. The results of a study are presented here:
Culture test
Dipstick test Positive Negative Total
Positive 84 43 127
Negative 10 92 102
Total 94 135 229
(a) What is the sensitivity of the dipstick test in this study? What is its specificity?
(b) For a population in which the prevalence of urinary tract infections is 0.12, calculate the probability that an individual has a UTI given that he or she tested positive using the dipstick test?
(c) What is the predictive value of a negative test?
Explanation / Answer
a) Sensitivity = (true positive / Total infected)x100
= (84/94)x100 = 89.4%
Specificity = (true negative/total non infected) = (92/135)x100 = 68.15%
b) Prevalence = 0.12
P(individual has UTI | tested positive)
= (0.12x0.894)/(0.12x0.894 + (1-0.12)(1-0.6815)
= 0.1073/(0.1073 + 0.2803)
= 0.2768
c) Negative predictive vlue = P(no disease | negative result) = 92/102 = 0.902 = 90.2%
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